CELP type speech codec often uses a post filter to improve the subjective quality of decoded speech (for example, see Non-Patent Document 1). The post filter in Non-Patent Document 1 is based on serial connection of three filters of formant emphasis post filter, pitch emphasis post filter and spectrum tilt compensation (or high band enhancement) filter. The formant emphasis filter makes the valleys in the spectrum of a speech signal steeper, and thereby provides an effect of making quantization noise, which exists in the valley portion of the spectrum, hard to hear. The pitch emphasis post filter makes the valleys in the spectral harmonics of a speech signal steeper, and thereby provides an effect of making quantization noise, which exists in the valley portion of the harmonics, hard to hear. The spectral tilt compensation filter mainly plays a role of restoring the spectral tilt, which is modified by the formant emphasis filter, to the original tilt. For example, if the higher band is attenuated by the formant emphasis filter, the spectral tilt compensation filter performs high-band emphasis.
On the other hand, in a decoded signal in CELP type speech codec, components of higher frequency are more likely to be attenuated. This is because waveforms matching is more difficult for signal waveforms of high frequencies than signal waveforms of low frequencies. This energy attenuation of the high-band components of a decoded signal gives to listeners an impression that the band of the decoded signal is narrowed, and this causes the degradation of subjective quality of the decoded signal.
To solve the above-described problem, a technique of performing a tilt compensation of decoded excitation signals is suggested as post processing for decoded excitation signals (e.g. see Patent Document 1). With this technique, the tilt of a decoded excitation signal is compensated based on the spectral tilt of the decoded excitation signal such that the spectrum of the decoded signal becomes flat.
However, if high-band emphasis is performed excessively upon performing tilt compensation of the speech excitation signals as post processing for decoded excitation signals, quantization noise, which exists in the higher band, is perceivable, which may degrade subjective quality. Whether this quantization noise is perceived as degradation of subjective quality depends on the features of a decoded signal or input signal. For example, if the decoded signal is a clean speech signal without background noise, that is, if the input signal is such a speech signal, quantization noise in the higher band amplified by high-band emphasis is relatively more perceivable. By contrast, if the decoded signal is a speech signal with high-level background noise, that is, if the input signal is such a speech signal, quantization noise in the higher band amplified by high-band emphasis is masked by the background noise and is therefore relatively hard to be perceived. By this means, if the background noise level is high and high-band emphasis is too little, giving an impression of a narrowed band is likely to cause the degradation of subjective quality, and therefore sufficient high-band emphasis needs to be performed.    Non-Patent Document 1: J-H. Chen and A. Gersho, “Adaptive Postfiltering for Quality Enhancement of Coded Speech,” IEEE Trans. on Speech and Audio Process. vol. 3, no. 1, January 1995    Patent Document 1: U.S. Pat. No. 6,385,573